Heart disease is the number one killer in the U.S. and many other countries. In the United States, heart disease results in the death of almost one million people per year. The high mortality and morbidity rate has led to many drug and device therapies to intervene in the progression of heart disease. Aggressive therapy for many forms of heart disease involve interventions where a cardiologist inserts a catheter in the patients artery or vein and performs procedures such as angioplasty, pacemaker or implantable defibrillator lead insertion or electrical mapping. These procedures have grown dramatically on a cost-basis: 947 million dollars were spent in 1990 vs.4.6 billion dollars spent in 1996.
Interventional procedures in cardiology are all the more remarkable since these procedures are performed only under radiographic guidance. Radiography presents the physician with a faint outline of the heart and its relation to the catheter. While radiography provides the cardiologist a crude guide, it does not allow examination of surfaces of the heart and vasculature or provide enough vision to guide procedures such as angioplasty or ablation.
In other body cavities, not filled with blood, such as the stomach or esophagus, fluid can be evacuated permitting visible wavelengths to be used in endoscope imaging. Visualizing the structure allows minimally invasive procedures such as ablating, stapling and suturing to be performed. These procedures, called laparoscopic procedures, are guided by the insertion of an endoscope, permitting visual examination of the treatment. These procedures are done in a saline bath or air to permit clear viewing. For example, minimally invasive orthopedic procedures rely on the endoscopic image to guide treatment. It is unfortunate cardiology has not had access to this technology since the common procedures would benefit from visualization.
The advantages to seeing structures in the cardiovascular system are numerous. Current methods of visualizing structures in the cardiovascular system are limited to radiography, ultrasound and angioscopy. Radiography is the standard visual tool used to image interventional cardiology procedures. It is applied by a large X-ray apparatus on a C-arm that will rotate around the patient through 180 degrees. The heart appears as a faint outline; while the metallic catheters are brightest. This allows for gross estimation of the catheter end to faint landmarks of the heart. The C-arm is frequently repositioned to give better viewing perspectives. Once the catheter has been navigated to the heart it can be placed in a coronary artery. In a self-contained entity such as an artery or vein, flouroscopic sensitive dye can be injected out the distal end of the catheter and viewed on the radiography camera for a short distance before it diffuses with blood. This technique is used to spot constricted areas in the coronary arteries. It has been shown that radiography, however, usually underestimates the degree of stenosis and therefore is only useful in providing a gross measure of flow.
More accurate assessments of coronary flow have been pioneered in the coronary arteries to evaluate angioplasty treatment. In the vasculature, the current angioplasty procedure for revascularization of an occluded coronary artery is to insert a catheter in the arterial tree, select the appropriate coronary artery, place an expandable balloon across the lesion and apply external pressure. As the pressure is reduced an expandable metallic structure (stent) remains opened to provide a scaffold, preventing the coronary artery from closing. This procedure is only effective long-term about 75-80% of the time. It is thought that many of these restenosis are due to inappropriate pressure application or inadequate stent placement. Oftentimes, postmortems have revealed stent buckling which can obstruct the flow rate in the coronary artery.
This information is so important, a form of endoscopy for the coronary arteries has been developed; called angioscopy. Examples of the art are contained in U.S. Patents since these devices operate in the visible spectrum, the blood must be removed and replaced with saline to permit viewing. Since blood is opaque at visible wavelengths, angioscopy only works when the blood is pumped out of the artery and replaced with clear saline solution. As stated in Arterial Imaging: Text and Atlas (White, D. M., Chapman and Hall, 1993), "In order to obtain adequate visualization within the vessel lumen, blood must be removed from the field of vision as even small amounts of red cells can obscure the clarity of the image." In angioscopy, the catheter is directed to the arterial segment of interest and two occluding balloons are pressurized allowing the intervening blood to be removed and replaced with saline. An angioscopic catheter requires multiple ports: fluid pressure ports, an irrigation port and a port for the endoscope. Consequently, the devices are difficult to operate, since the physician must position the catheter, activate distal and proximal balloons, extract the blood from a port between the balloon and replace with saline. This cumbersome procedure, developed in the 1980's, has been used infrequently since it was very time consuming and presents a danger to the patient. The bulkiness of the angioscopic catheter, the complicated procedure and the inherent risk to the patient in having an artery totally occluded for the time of the procedure has made this procedure unpopular and relegated it to a few research-oriented hospitals. The disappointment with this technology has led to the development of a catheter ultrasonic technique called intraluminal ultrasound.
In an effort to produce visualization at the site of angioplasty for the surgeons, intraluminal ultrasound (for example, U.S. Pat. No. 4,917,097) devices have been designed. The intraluminal device is a modification of the familiar external ultrasound device used to visualize prenatal infants and heart valves. External ultrasound devices only have resolutions in the centimeter region. Greater resolution requires a higher greater frequency. The physics of the instrument dictate that the higher the frequency of the ultrasound transducer, the greater the potential for higher resolution and concomitant shorter penetration through the tissue. Higher frequencies do not penetrate as far requiring the transducers to be very near the structure. To visualize angioplasty procedures the resolution needs to be about 0.2 mm, requiring a 20 MHz device. A 20 MHz device will only penetrate about 1 cm of tissue before it is drowned in background noise. Consequently, for application in the coronary vasculature most of the device must be miniaturized so it can be inserted in the artery close to the blockage area. At a frequency of 20 MHz, it is possible to view the structures of the coronary artery only within a centimeter distance, requiring the transducers to be inserted in the artery. In one embodiment (U.S. Pat. No. 4,917,097) of this technology, a multitude of ultrasonic transducer crystals (64) are placed on the end an around the circumference of a 1.2 mm catheter to produce a visual view of the site of angioplasty. The catheter's construction is bulky because both the transducers and three integrated circuit signal processing chips have to be placed on the catheter tip. It is necessary to process the small signal with as little transmission through conductive wires. The positions of the electrical driver components (being external rather than internal) will generate ambient electronic noise, which contributes to the limitation of resolution from the catheter. The resultant picture is of marginal resolution quality because of the limited number or density of transducers, which corresponds to a 64-pixel image. The geometry of the catheter allows each pixel approximately 6 degree field-of-view of the wall of an artery. If the artery inner diameter were 5 mm, then each pixel would view 0.26 mm of the wall. This assumes there is not overlap of coverage by each pixel and there is no ambient noise in receiving the signal. Unlike light, which reflects off of surfaces, ultrasound is also absorbed to a significant degree by body tissue and then reflected; resulting in fuzzy or overlapping tissue interfaces in the image. Also, since the received ultrasonic signal produces only microvolts of response. At these frequencies, it is just above ambient noise. It is difficult to process a clear signal of this size when the system noise is very close to this amplitude. Thus the poor quality of the ultrasonic image is due to (1) small number of ultrasound transducers (2) absorption by internal tissue and (3) low signal to noise ratio. The arrangement of the receiving pixels and the transmitter produces a blind spot in the first 0.2-0.3 mm of the image. This blind spot causes difficulty in the visual interpretation of the image produced by this device
Another embodiment that is used to view the coronary arteries is the IVUS catheter. It consists of a single spinning piezoelectric transducer, which operates on a sector scanning principle to produce its forward views. A stylet connected to the transducer is spun to provide images over a full circle. Side by side comparisons show similar results for this system and the one discussed above. Either approach is a poor substitute for optical pictures. With optical systems, a pixel could correspond to a single fiber optic bundle of which there are thousands in conventional endoscopes. Tissue surface definition is well-defined, since light scatters off of surfaces instead of being absorption by the tissue. Consequently, optical and ultrasound images are not comparable.
With the interest in electrical mapping and catheter ablation, cardiologists specializing in these procedures, called electrophysiologists, have searched for visualization techniques to assist them in these procedures. In these procedures, catheters are inserted to precise positions within the heart. Any visualization of these procedures would be extremely valuable. Researchers have focused on intra-cavitary ultrasound, a technology similar to intra-luminal ultrasound, but at lower wavelengths to see greater distances. Bom (U.S. Pat. No. 3,938,502), describes a crude ultrasound device for use in the heart and blood vessels. Like its vascular counterpart, this technology suffers from the inherent problems of ultrasound; poor resolution and insufficient differentiation of tissue surfaces. Obviously, there is no heart analog for angioscopy, since it is not feasible to replace blood inside the heart with transparent saline solution.
There is no known prior art on an infrared endoscope illuminating a structure through blood with infrared light and observing the returning reflected light from the structure. In medicine, infrared imaging is used in a very different manner; detecting cancerous cells which have different temperature, by measuring the emitted infrared radiation from the cell.
Infrared technology is principally used in medicine, in a different manner from the present invention; to identify abnormal or cancerous cells by measuring emitted radiation from body structures; a field called thermography. All warm or ambient objects radiate energy in the infrared, peaking at 10 microns, with measurable radiation seen as low as 2 microns. Abnormal cells radiate slightly differently and are therefore seen as objects of distinct color. The most common usage is mammography. When breast tissue is photographed with an infrared camera, cancerous tissue shows up as a different color indicating tissue of different temperature at that point. This principle has been applied to other parts of the anatomy, such as skin cancer. Additionally, endoscopes are disclosed in the art to measure similar cancer characteristics in internal body cavities.
With one exception discussed below, infrared endoscope art is limited to imaging the emitted infrared spectrum internal body cavities; no art has been found on internally illuminating the body cavity with infrared light. The endoscope needs to view the internal body cavity through air, since water is not transparent to emitted infrared radiation for most wavelengths. Adachi (U.S. Pat. No. 5,445,157) describes a thermographic endoscope sensing emitted infrared radiation from 9-11 microns, for the purpose of imaging temperature differences in abnormal cells such as cancerous cells in a gaseous medium. The image, corresponding to small temperature differences, is enhanced by means of injecting low-temperature gas out of the distal end of the endoscope. This device could not be used in a fluid medium such as blood because of the extremely high absorption values of blood at these wavelengths as discussed in the "Absorption of Water" section below.
Bonnell (U.S. Pat. No. 5,711,755) describes a means of imaging infrared radiation emitted from interior structures with an endpscope in the 2-14 micron region and combining it with a visual spectrum image for detection of abnormal cells such as gallstones. A preferred embodiment involves the use of a cooling fluid to further enhance the temperature discrimination sensitivity. This device would only "see" abnormal cells; ones that radiate at a slightly different temperature from the surrounding tissue. Normal cells radiate at the same temperature and would be filtered out through electronic processing and therefore could not be imaged. This patent teaches that using the visual spectrum image overlaid with the infrared spectrum image creates a composite image where cells of abnormal temperature appear as objects of different contrast on the visual image. This patent is concerned with detecting only emitted radiation, as there is no illuminating infrared light source. No mention is made of detecting this emitted radiation anywhere in the cardiovascular system (i.e. blood media). In fact, detection in a blood media would smear out temperature differences since the emitted radiation is so small. An infrared picture of the emitted radiation of a structure, through blood would not contain any image--even if abnormal cells existed.
Viewing a normal structure in a fluid media requires illumination at a higher power level than the normal emitted background, to produce sufficient reflectance for optical imaging. The emitted radiation detected in U.S. Pat. Nos. 5,445,157 and 5,711,755 are background radiation which obscures the image and requires light sources with higher wattage to "flood" the field. There is no art found using illumination at infrared wavelengths through a fiber optic bundle to produce images of an internal body structure using the reflected and scattered illumination collected by an endoscope.
Nakamura (U.S. Pat. No. 4,953,539) describes an endoscopic imaging device placed in an organic body and illuminated external to the body with infrared radiation in an effort to visualize the reverse surface of internal organs such as the bladder. Nakamura teaches that if an organ is backlit externally with infrared light, the reverse surface can be visualized with an endoscope inserted internal to the organ and sensitive to at least infrared light. Infrared light is chosen as the illuminating source since tissue has lower permissivity in the infrared region; it penetrates further through tissue. With this arrangement, the reverse surface of the tissue is said to be visualized. There is no teaching, references, or prior art citations referring to visualizing structures through opaque fluids such as blood. The Nakamura teaching is illuminating the organ externally with infrared light, and is not relevant to internally illuminating with infrared light at particular wavelengths which render liquid with suspended particles such as blood semi-transparent.
Unrelated to thermography and infrared imaging, Boutacoff (U.S. Pat. No. 5,147,354) describes a mid-infrared endoscope that is used to deliver or transmit infrared energy for laser surgeries. He discloses an endoscope operating between 1.8-2.2 microns which corresponds to commonly used lasers which have wavelengths in that region for the purpose of ablating and welding internal structures such as within the human knee. There is no suggestion of imaging structures in the infrared region. The wavelength spectrum was chosen at a water absorption peak (where water is more opaque), creating more localized heat which is used in welding and ablating tissue. This would be entirely unsuitable if applied to the present invention. Boutacoff refers to an endoscope suitable for transmission of infrared energy of commonly used lasers; no mention is made of imaging structures with infrared energy.
The prior art presented on infrared endoscopes does not disclose or suggest using internal infrared illumination through a fiberoptic cable, to illuminate structures in an opaque-body-fluid environment, such as blood, and viewing the scattered light from the structure to form an image.